“Seismic Design and Rehabilitation of Buildings” Conference, Tbilisi, May 31, 2014

An Overview for Practicing Engineers

SEISMIC REHAB OF RC STRUCTURES

A. T. TankutMiddle East Technical University

Ankara, Turkey

FOREWORD

Of my two alternative proposals, namely, - A report on recent scientific research or - An overview for practicing engineers, the latter was preferred. Thus,

In this talk, existing information will be presented in a classified manner.

Nothing sophisticated, nothing impressive...

OUTLINE

Introduction

Rehabilitation Strategy

Rehabilitation Technologies

OFR-Occupant Friendly Rehabilitation (if time allows)

Concluding Remark

INTRODUCTION

INTRODUCTION

Earthquake is a natural phenomenon

It is tolerable in countries where people and built environment are prepared for it;

It leads to a disaster in countries where built environment is not prepared for it and people just wait and do nothing.

INTRODUCTION

Earthquake Preparedness consists of

Disaster Management (post-quake) – Search & rescue, sheltering, food, medical care etc. (Tip of the iceberg, easy but ineffective)

Risk Management (pre-quake) – Safe towns, safe structures, well educated public, well trained engineers, effective financing etc. (Body of the iceberg, hard but very effective)

INTRODUCTION

Risk Mitigation is a multi-dimensional activity having various aspects such as,

- Earth sciences - Urban planning- Civil engineering - Environmental- Educational - Social - Psychological - Administrative- Financial - Economic- Legislative - Political etc.

INTRODUCTION

A huge seismically unsafe building stock

A systematic assessment will reveal that - A small number seismically safe, - A certain portion to be demolished, - The majority to be rehabilitated. So,

Seismic rehabilitation is one of the most critical aspects of risk mitigation.

INTRODUCTION

Risk mitigation unavoidably requires

A well definedDamage Mitigation Strategy

A realistic and well organised Action Plan

A consistent and insistent implementation for decades

And plenty of political will (???)

REHAB STRATEGY

REHAB STRATEGY

Development of an efficient rehabilitation strategy requires careful considerations of

Common deficiencies observed;

Performance levels to aim at;

Rehab technologies available.

Common Deficiencies

COMMON DEFICIENCIES

RC framed building structures with hollow brick masonry infill are common in SEE.

Low-rise (12 floor) are not vulnerable;

High-rise (> 1012 floor) buildings are carefully designed and constructed;

Mid-rise (38floor) bldgs of inferior material, design & construction quality are a problem.

COMMON DEFICIENCIES

Mid-rise buildings of inferior quality

Constitute the majority in small towns;

Collapse in the pancake mode; thus

Are responsible from the high number of human losses and severe damage,

Are generally too good for demolition;

Are greatly in need of rehabilitation.

COMMON DEFICIENCIES

Common deficiencies of such buildings:

Insufficient lateral stiffness

Deficient reinforcement detailing- Insufficient confinement & anchorage - Inadequate joint reinforcement etc.

Deficient design practice- Horizontal/vertical irregularities- Short columns; soft storeys etc.

Poor concrete; poor workmanship etc.

Performance Levels

PERFORMANCE LEVELS

Generally accepted performance levels:

Functional – Slight or no damage (in the

code earthquake); continued serviceability

Immediate occupancy – Light damage; serviceability after inspection

Life safety – Moderate damage

Collapse prevention – Severe damage, no collapse, no casualties

PERFORMANCE LEVELS

Most of the current seismic codes

Were developed for new structures;

Aim at a performance level above life safety without explicitly mentioning;

Apply to repair and strengtheningbesides new construction;

No flexibility for “performance level” and “remaining service life” considerations.

PERFORMANCE LEVELS

Special code provisions are needed for rehabilitation providing flexibility for- Performance level and- Remaining service life considerations.

Designer should be given the choice of - Life safety or collapse prevention- 20 or 40 or >60 yrs service life.

Rehab Technologies Available

REHAB TECHNOLOGY

Member strengthening techniques are available for

Columns (axial load & bending)

Beams (bending & shear)

Beam-column joints (shear)

Slabs (diaphragm action)

REHAB TECHNOLOGY

System behaviour improvement techniques are also available

Lateral stiffness increasing elements (To relieve members from seismic effects)

Base isolation, dampers etc. (To minimise seis action transfer to structure)

Rehab Strategy Proposed

REHAB STRATEGY

Member strengthening is preferred when - Structural weakness is localised or- A small number of members are deficient.

Member strengthening is not feasible when - Deficiencies are widespread and- Lateral stiffness is insufficient

REHAB STRATEGY

Considering common deficiencies above a sensible strategy can be formulated as;

System behaviour improvement is essential and should be accompanied by

Strengthening of a limited number of deficient members

REHAB TECHNOLOGIES CLASSIFIED

System Improvement Techniques

SYSTEM IMPROVEMENT

Lateral stiffness increasing elements such as

Cast-in-place reinf conc infilled frames

Steel cross bracing

Post tensioning

External rigid frame to support the str

Masonry infills converted to shear walls

Member Strengthening Techniques

COLUMN STRENGTHENING

Reinforced concrete jacketing

Effective for axial load, but complicated and not recommended for bending

Full jacket is best, partial is acceptable

Well confinement in jacket is essential

Bar welding is recommended

COLUMN STRENGTHENING

Steel jacketing

Only for axial load, never for bending

Tight connection with base plates and

Well confinement are essential

COLUMN STRENGTHENING

CFRP confinement

Effective as confinement especially in circular columns; to a lesser extent in rectangular ones

Effective to improve lap splice performance and capacity

BEAM STRENGTHENING

Additional layers with new steel

Effective for bending

Bar development is critical

Welding is advisable

Stirrups or Z-bars are essential

CFRP applications to the same effect are also possible.

BEAM STRENGTHENING

External clamps as shear reinforcement

Effective for shear

Limited prestressing is recommended

CFRP applications to the same effect are also possible.

BEAM STRENGTHENING

Beams connected to new lateral stiffness elements become “coupling beams” and receive enormous bending and shear.

Hinging is unavoidable. Make sure,

It is properly confined to tolerate hinging,

Shear capacity is higher than bending cap.

JOINT STRENGTHENING

Joints are critical under seismic action, and they are generally deficient (Required confinement is not usually provided)

Effective and practical strengthening techniques are not yet available(Suggestions are ineffective or impractical).

Another reason to endorse the system behaviour improvement approach

SLAB STRENGTHENING

Major contribution of the slab to the seismic performance is diaphragm action.

Additional layers with new steel

Effective for bending & in-plane stress

Rough connection surface and

Shear connectors are essential

Deformation recovery is not recommended

OCCUPANT FRIENDLY REHABILITATION

BASIC QUESTION

Cast-in-place RC infilled frame technique is suitable for post-quake repair of the evacuated buildings;

But it is not suitable for pre-quake rehabilitation of the buildings still in use.

Suitable techniques should be developed.

THE CHALLANGE

To develop a rehabilitation method,

Suitable for the common local building type (Hollow brick infilled RC frame)

Practical & economical, and above all

Occupant friendly (no more disturbance than an ordinary painting job)

THE ANSWER

The answer is OFR (occupant friendly rehab)

To reinforce existing masonry infill wall with epoxy bonded PC panels, which are,

- Light enough to be handled by two men

- Relatively thin, 40~50 mm (high strength)

- Connected to infill wall by epoxy, and to frame by epoxy bonded dowels

THE IDEA

Cast-in-place reinforced concrete infill is known to improve the seismic structural performance.

Why shouldn’t PC panel reinforced masonry infill do the same ?

TEST FRAMES

1/3 scale, one-bay, two-storey inferior quality RC frames,(representing the actual practice)

- Strong beam-weak column

- Insufficient confinement

- Low quality concrete (C13~C16)

REFERENCE

-200

-150

-100

-50

0

50

100

150

200

-50 -40 -30 -20 -10 0 10 20 30 40 50

a. Test PR

Lat

eral

Lo

ad (

kN

)

Top displacement (mm)

STRENGTHENED (SQUARE)

-200

-150

-100

-50

0

50

100

150

200

-50 -40 -30 -20 -10 0 10 20 30 40 50

b. Test PI1

Lat

eral

Loa

d (k

N)

Top displacement (mm)

STRENGTHENED (STRIP)

-200

-150

-100

-50

0

50

100

150

200

-50 -40 -30 -20 -10 0 10 20 30 40 50

c. Test PI2

Lat

eral

Loa

d (k

N)

Top displacement

(mm)

REFERENCE

STRENGTHENED (SQUARE)

STRENGTHENED (STRIP)

ENVELOPES

-250

-200

-150

-100

-50

0

50

100

150

200

250

-50 -40 -30 -20 -10 0 10 20 30 40 50Top Displacement

(mm)Lat

eral

Load

(k

N)

PR

PI1 PI2

PERFORMANCE IMPROVEMENT

Relative to masonry Relative toinfilled frame bare frame

Lateral load capacity 2.5 times 15 times

Lateral stiffness 3 times 20 times

Ductility 2 times 0.2 times

Energy dissipation 3 times 60 times

INTERPRETATION

Significantly improved performance:

Increased load carrying capacity

Increased initial & final stiffness

Delayed strength degradation

Decelerated stiffness degradation

Better ductility

Much higher energy dissipation

CONCLUSION

PC panel technique is an effective & practical seis rehab method for existing buildings.

Leads to a significant improvement in seismic performance

Is easily applied to buildings in use with minimal disturbance

Is cost effective (Comparable to cast-in-place RC infills)

CONCLUDING REMARK

A REGRET

The speaker regrets

For not being able to present the manuscript of this talk to be included in the proceedings.

He was unable to finalise the manuscript in the “8 days” he was given by the authorities.

Lazy old man!..

THANKSfor your attention...

“Seismic Design and Rehabilitation of Buildings” Conference, Tbilisi, May 31, 2014

An Overview for Practicing Engineers

SEISMIC REHAB OF RC STRUCTURES

A. T. TankutMiddle East Technical University

Ankara, Turkey